A finger print sensor has been conventionally employed as an individual identification device, and a requirement for the finger print sensor is detection of a finger print with simplicity and good precision. Various types of finger print sensors of this kind have been studied and developed, which include a type to detect a finger print optically, a type to detect a finger print electrically, and the like. JP-A 09-126918 (1997) and JP-A 10-300610 (1998) describe, for example, that microsensor sections having electrodes are arranged in a matrix, wherein a pressure from a finger is converted into an electric signal to thereby detect a finger print. The microsensor sections each are constructed such that two electrodes are disposed opposite each other with a cavity present therebetween.
FIG. 16 is a sectional view of a microsensor section in fabrication process. An etching barrier layer 102 is stacked on a silicon substrate 101 and a first metal layer 103 made of Au or Ti is formed thereon in a predetermined pattern. The first metal layer 103 is used as a first electrode of a variable capacitor or a first terminal of a microcontactor. A separation film 104 made of polycrystalline silicon or Al is formed on and covers the first metal layer 103, and a second metal layer 105 made of Au or Ti is formed on the separation film 104. An insulating film 106 made of silicon nitride covers all over the surface of the substrate 101. An opening 107 reaching as far as the separation film 104 is formed on the surface of the microsensor section through the second metal layer 105 and the insulating film 106 to expose the separation film 104 to the outside in the opening 107. Note that this state is shown in FIG. 16. Thereafter, wet etching is applied to the substrate 101, in which a solution etches the separation film 104 made of polycrystalline silicon or Al to remove the separation film 104 and to form a cavity. After the etching, the opening 107 is closed with silicon nitride or the like to hermetically seal the cavity. When a pressure from a finger is imposed on the microsensor, the insulating film 106 and the second metal layer 105 is curved to the first metal layer 103 side depending on the pressure, to which situation an electric signal is outputted in response to thereby detect a pattern of a finger print.
The second metal layer 105 serving as the upper electrode of a microsensor requires a flexibility to curve itself to the first metal layer 103 side depending on a pressure of a finger and a restoring force to restore its deformed state to the original one when the pressure does not act any longer. Despite such a requirement, in a case where the insulating film 106 covers the second metal layer 105, which has been conventionally adopted, differences in flexibility and elasticity between the second metal layer 105 and the insulating film 106 are large since the insulating film 106 is harder as compared with a metal, resulting in a high possibility that the second metal layer 105 or the like is broken during usage. Since, especially, an insulating film which has a large thickness and plural insulating films are formed on the second metal layer 105 in order to ensure an insulating property, a large difference in flexibility occurs between the insulating film 106 and the second metal layer 105.
With increase in the number of usage times of a microsensor, the second metal layer 105 is easy to be broken to thereby reduce a precision and durability since the second metal layer 105 having a flexibility is low in strength. Because the conventional second metal layer 105 is formed flat on the whole, a flexibility and a restoring force are set by selecting a material or a thickness thereof. If a film thickness is reduced in order to obtain a flexibility, however, a restoring force is weakened, while if a film thickness is increased in order to enhance a restoring force, a flexibility is lost; therefore, it has been difficult to achieve the second meal layer 105 having sufficient flexibility and sufficient restoring force in a conventional technology.